The genetic material of killer, an extrachromosomal element in Saccharomyces cerevisiae, has been correlated with the presence of several double-stranded RNA pieces. Genetic evidence suggests that a particular segment controls production of the toxin excreted by killer strains. The question still remains, however, as to whether this segment codes directly for toxin or for some function necessary for toxin expression. To answer this question, a screen has been devised to detect nonsense mutants in the genes controlling toxin production. This screen takes advantage of a temperature-sensitive nonsense suppressor to seek a strain that kills only at the permissive temperature. A battery of genetic tests will determine whether such strains are carrying a nonsense mutation and whether it maps on the particular RNA segment. Biochemical analyses will then be performed to seek nonsense fragments of the toxin. A peptide fragment of the toxin is expected if the nonsense mutation is in the gene coding for it. This approach is feasible to screen for nonsense mutants in all functions coded by the double-stranded RNA genome. Furthermore, because yeast is a eucaryotic organism that can apparently host alien viral systems, it may be possible to determine functions of other RNA viruses.